1. Field of the Invention
The present invention relates to a blue electroluminescent polymer, and to an organic electroluminescence device using the same. More particularly, the invention relates to a blue electroluminescent polymer comprising biphenyl units in a main chain of polyarylene, and to an organic electroluminescence device using the blue electroluminescent polymer.
2. Description of the Related Art
As the first electroluminescent device using organic material, C. W. Tang from Eastman Kodak company reported a device having a multilayered structure in which each layer has a different function. Since the multilayered structure device has advantages including lightness, thinness, various colors, fast switching speed and high luminance under low driving voltage, many studies on such multilayered devices have been ongoing in the past decade. As a result, great advances in the performance of the device, e.g., a balanced charge injection by using the multilayered structure, color tuning and improved quantum efficiency by a doping process, and a new electrode material made of an alloy, etc., have been developed in a relatively short period of time.
An organic electroluminescence display typically is classified either as a device using low molecular weight compounds, or a device using polymers, in terms of material and manufacturing process thereof. Low molecular weight compounds usually are layered on a device by a vacuum deposition process. The device using a low molecular weight compound is advantageous in that the purification of materials is easy, high purity thereof is possible, and color pixels can be easily attained. However, for practical use of the device, there still exist many items that can be improved to improve the performance of the device in terms of quantum efficiency, thermal/mechanical stability of thin film, color purity, lifetime etc.
Various studies on electroluminescent displays using low molecular weight compounds have been undertaken in many countries, especially in Japan and U.S.A. For example, Idemitsu-Kosan Co., Ltd.(Japan) first exhibited a 10 inch full color organo-electroluminescent display manufactured in a color mode using a color changing medium in 1997. Pioneer Corporation (Japan) presented a 5 inch PM (Passive Matrix) full color organic electroluminescence display. Recently, Pioneer Corporation and Motorola Inc. have arrived at an agreement concerning the mass production of cellular phones employing the organic electroluminescence display in a terminal. It suggests that the commercialization of the electroluminescent display using low molecular weight compounds will be possible soon.
On the other hand, studies for developing electroluminescent devices using polymers have been carried out intensively since a Cambridge group reported in 1990 that light was emitted when electricity was applied to poly(1,4-phenylene vinylene)(PPV), which is a π-conjugated polymer. Generally, π-conjugated polymers have an alternating structure of single bonds (σ-bonds) and double bonds (π-bonds) so that the polymer has delocalized π-electrons capable of freely moving along the polymer chain. The π-Conjugated polymer has semiconductive properties, and visible light corresponding to the HOMO-LUMO energy band-gap of polymers can be easily obtained in the range of whole UV-Visible spectra region, through molecular design of the π-conjugated polymer when the polymer is employed in a light-emitting layer of the electroluminescent device. In addition, thin films of the polymer can simply be formed on the device by a spin coating or a printing process. Accordingly, the manufacturing process of the device is very simple and cost-effective. Furthermore, the mechanical properties of the thin film of polymer are excellent due to its high glass transition temperature (Tg). Therefore, it is expected that devices using polymers have more advantages from a commercial point of view than the electroluminescent display using low molecular weight compounds.
However, devices using polymers have problems in that their luminance is lower than the electroluminescent device using low molecular weight compounds, and their durability is weak due to the deterioration of luminescent polymer. The deterioration of polymer can be caused by defects present in the polymer chain, which can occur during the synthesis of the polymer. In addition, the purification of polymer is difficult so that polymer of high purity is not easily obtainable.
In order to overcome these problems, a polymerization process capable of minimizing defects in polymer, and a purification process capable of removing impurities present in polymer, are needed. The polymer prepared by these processes can further improve the performance of the device using polymer.
The description herein of various problems and disadvantages of known apparatus, compounds, and methods is in no way intended to limit the scope of the invention. Indeed, various aspects of the invention may include some of the known apparatus, compounds, and/or methods without suffering from the same problems or disadvantages.